The present invention relates generally to the field of dental instruments. More particularly, the invention relates to those instruments used in the practice of endodontia to remove dead or damaged tissue from a tooth root canal preparatory to filling the root canal.
Extraction of a tooth may often be avoided by removing decayed, damaged, or dead tissue from the nerve canal within a tooth root. Typically, to remove decayed, injured or dead tissue from the root canal of a tooth, an endodontist will first drill into the tooth to locate the root canal and thereafter use instruments of small diameter, such as reams and files, to remove the decayed, injured or dead tissue from the nerve canal. The objective of tissue removal with the instrument is to produce a funnel-shaped canal, with the smaller end at the apical foramen, so as to provide the canal with a desirable form for filling. Such root canals are not necessarily straight. The instrument must be able to follow a curved path as it is moved through the canal for purposes of removing the tissue. Therefore, the instrument must be flexible yet possess sufficient strength so that it is not easily broken when stroked and rotated through a root canal.
Commonly, both files and reamers have flutes which spiral along a portion of the instrument length. Whether the instrument is considered to be a file or a reamer is determined by the pitch of the flutes. A greater pitch enables the instrument to cut better in a rotary mode. Hence, a large pitch instrument is characterized as a reamer. A lesser pitch to the endodontic instrument flutes enables the instrument to cut better in the reciprocating mode of motion. A low pitch instrument is therefore characterized as a file. During a cleaning and shaping operation performed with an endodontic file or reamer, the instrument is normally rotated and reciprocated into and out of the root canal along the longitudinal axis of the instrument. Therefore, even during rotational movement of a reamer through a canal the instrument is commonly reciprocated to a degree to effect the desired cut. Similarly, a file is commonly rotated slightly as it is reciprocated longitudinally.
In order for the instrument to extirpate root canal tissue as it is rotated and moved longitudinally, relatively high torsion forces must be typically applied to the instrument. These high torsional loads on the instrument may lead to torsional failure of the instrument. In addition, the curved path of common root canals requires that the instrument concurrently be subject to bending and torsional loads as it is moved along the canal. This combination of bending and torsional loads as the instrument is moved along the canal increase the likelihood of torsional failure.
Traditional ISO Standard files for endodontia have a taper angle of about two percent. This taper is usually continuous over the full working length of the file. Size of the file is based upon the file tip diameter. Using ISO Standard files, a "crown down" technique of radicular extirpation begins by first penetrating the root canal with a large diameter instrument followed sequentially by one or more files of diminishing diameter until the apical foramen is reached. As the root canal bore is extended toward the root apex, the active cutting proportion of the file working length increases while, at the same time, the file size and tip diameter is diminishing. This increase in the active cutting portion of the file necessarily results in an increase in the torque stress imposed on the file and on the probability of file failure.
Another approach for radicular extirpation is based upon use of several files having the same maximum diameter but with a graduated series of decreasing taper angles from the file tip that terminate at the maximum diameter. The canal is started with the file of greatest taper angle. The first file is advanced to a depth corresponding to the desired entrance diameter. The second file, having a smaller angle of tapered convergence from the tip to the maximum diameter, has a greater axial length from the maximum diameter base to the file tip. Hence, the second file will advance the canal depth from the same entrance diameter. This procedural sequence will continue down to the apical foramen. Similar to the "crown down" procedure, the decreasing taper angle method of extirpation imposes greater torque on the most fragile files.
It is therefore, an object of the present invention to provide an improved method of radicular extirpation that operatively limits the magnitude of torque imposed on a root canal file.
Another object of the invention is a set of root canal instruments for carrying out the improved method of the invention.
A further object of the invention is a root canal instrument set that operatively limits the magnitude of torque that is likely to be imposed on each instrument in the set.